A device for measuring forces that displace a capacitor plate with respect to one or more fixed capacitor plates is shown in U.S. Pat. No. 3,911,738, issued to Fischer. As a force displaces the movable plate, a responsive displacement signal is used to generate differential electric pulses that indicate the amplitude of the force being measured and that, when applied to the capacitor, electrostatically oppose the force and urge the movable plate toward its original position.
The Fischer apparatus uses an elaborate capacitive bridge circuit to measure voltage level changes caused by deflection of the plate. When the electrostatic counterbalancing force is generated to restore the plate, displacement is reduced by the generated counterbalancing force and thus, even more difficult to detect.
The use of analog voltage levels is susceptible to noise, especially switching noises inherent in the oscillator and phase-sensitive rectifier of the device. To improve the signal-to-noise ratio of the device, the movable plate must be sufficiently large to generate a detectable signal difference for small displacements. Furthermore, the noise susceptibility, particularly of the capacitive bridge circuit, substantially precludes system miniaturization; therefore, relatively expensive conventional hybrid technology must be used to fabricate the force-sensing elements. In addition, proper operation of the Fischer apparatus requires that it avoid rather than allow a pull-in condition.
Publication "The Resonant Gate Transistor" IEEE Trans. Electron Devices, Vol. Ed-14, pp. 117-133, 1967 by Nathanson et al. describes a pull-in phenomenon as an undesirable characteristic of a resonant gate transistor. The phenomenon is exhibited by a parallel-plate capacitor having one plate suspended with a spring and a fixed plate. An applied voltage across the two plates induces an electrostatic force which tends to pull the spring-supported plate to the fixed plate. The capacitor gap gradually decreases with increasing voltage until a threshold voltage at which the gap collapses and the plates remain latched until the applied voltage is removed. Resonant gate transistors are often used as a tuning device in an oscillator circuit, an application which would not benefit from the pull-in phenomenon.